Vibronic sensors are finding increased application in automation technology, especially in process measurements technology, and even in manufacturing technology. The oscillatable element of a vibronic sensor is connected with a membrane by material bonding and can be embodied as an oscillatory fork or as a single rod. The membrane and the oscillatable element connected with the membrane are excited to oscillate via a transmitting/receiving unit. The transmitting/receiving unit is usually at least one piezoelectric, respectively electromechanical, element. Moreover, also known are so-called membrane oscillators, in the case of which the oscillatable element is the membrane.
Usually, a vibronic sensor is excited to oscillate via an analog electronics, wherein the analog electronics forms together with the sensor the analog oscillatory circuit. Corresponding vibronic sensors, respectively vibronic measuring devices, are manufactured and sold by the company, Endress+Hauser in varied embodiments under the marks LIQUIPHANT and SOLIPHANT.
Vibronic sensors enable detection of a process- or system specific parameter, such as the limit level of a liquid or a solid in a container. Usually, for detection of a predetermined fill level (limit level), the sensor is operated with the resonant frequency of the oscillatable system. By detecting the frequency change at the set phase of 90°, it can be recognized whether the oscillatable unit is in contact with the medium or is oscillating freely.
Moreover, it is known to evaluate the oscillatory behavior of vibronic sensors in a medium in order to ascertain, respectively to monitor, process- and/or system specific parameters. Process specific parameters include especially density and viscosity, however, also temperature. For the purpose of determining the density of a liquid medium, the phase difference (often also referred to simply as phase) between the input signal and the output signal is set to 45° or 135°. In setting this phase difference, a frequency change is uniquely attributed to a change of the density of the medium, since an influence of the viscosity of the medium can be excluded. Published International Application WO 02/031471 A2 describes an apparatus for viscosity measurement. European Patent EP 2 041 529 B1 teaches an apparatus for determining the density of a liquid medium.
As evident from the above mentioned examples, an analog electronics has the disadvantage that it is relatively inflexible. Especially, the analog electronics must be matched to each sensor, respectively sensor type, as a function of its oscillation characteristics and further as a function of the respective application—thus whether the sensor is to be applied for fill level-, density- or viscosity measurement.